Purpose:To commission a new device for pre-treatment patient specific verification of VMAT plans, which is capable of reconstructing 3D dose with high resolution.

Methods:Octavius 4D system (PTW) consists of an ion chamber array embedded in a cylindrical phantom which, assisted by an inclinometer, rotates synchronously with the gantry. For VMAT plans, it measures planar dose distributions as a function of gantry angle in order to compute the resulting 3D dose distribution. There are two options for the ion chamber array: Octavius 729 (general purpose) and Octavius 1000 SRS (small field size, high resolution). We have used three linacs: two Varian Clinac iX and one TrueBeam with MLC HD120, and the TPS has been Eclipse (v.10.0). The commissioning measurements consisted of the following tests: (1) homogeneity in response; (2) linearity with dose; (3) reproducibility; (4) leakage current, pre- and post-irradiation; (5) verification of typical VMAT and SBRT plans, comparing TPS-calculated versus measured dose using 3D local gamma index (3%, 3 mm criteria).

Results:We found the following results for Octavius 729 (and, in brackets, for Octavius 1000 SRS): (1) Homogeneity between chambers was 1.9% (0.8%). (2) Excellent linearity for both cases, with r² value better than 0.999. (3) Reproducibility was 0.52% (0.08%). (4) Leakage increases with previously measured dose and shows its largest variation for large field sizes, fluctuating between -0.33 and +0.33 cGy/min (between -0.006 and +0.020 cGy/min). (5) Verifications of typical clinical plans, performed with the adequate array, showed a mean pass ratio of 89.3% (98.7%).

Conclusion:Octavius 4D system together with Octavius 729 is an adequate tool for VMAT patient specific QA. With Octavius 1000 SRS, this capability is extended to those treatments requiring better resolution, as is the case for SBRT.